Farmed salmon bring PCBs to the table.

High concentrations of chlorinated organic contaminants in farm-raised Atlantic salmon may warrant limiting consumption of the otherwise-healthful fish to no more than once per month, researchers say. They add that most wild-caught Pacific salmon can be eaten at least 2 or 4 times monthly without significantly increasing cancer risk.

Compared with wild salmon caught in the northern Pacific Ocean, salmon farmed in Europe and North America are significantly more contaminated with polychlorinated biphenyls (PCBs) and 13 other organic chemicals, report Ronald Hites of Indiana University in Bloomington and his colleagues, in the Jan. 9 Science. Salmon farmed in Chile contain PCBs and five of the other chemicals in higher average concentrations than wild salmon has, the study also found.

The researchers base their dietary advice on cancer-risk assessments that the U.S. Environmental Protection Agency has made for four of the contaminants, including PCBs. The U.S. government has not endorsed the consumption limits suggested by Hites' team. Nobody has yet evaluated whether the cancer risks from contaminants outweigh the benefits derived from consuming salmon's heart-helping fats, most notably the omega-3 fatty acids.

Hites and his colleagues used gas chromatography-mass spectrometry to analyze salmon samples culled from nearly 600 fish of various wild and farmed varieties, plus 144 farm-produced fillets.

The researchers also analyzed contaminants in commercial salmon feeds from Chile, Europe, and North America, since diet is the fish's main source of chlorinated contaminants. They found the heaviest contamination of both fish and feed in Europe.

Rare 20 years ago, farmed Atlantic salmon now account for the majority of salmon available in supermarkets. Globally, more than 1 billion kilograms are farmed each year.